1. Field of the Invention
The present invention relates to a torque sensor for use in, for example, a power steering apparatus for an automobile.
2. Description of the Prior Art
FIG. 5 is a perspective view showing a conventional torque sensor of the foregoing type. As shown in FIG. 5, a steering shaft is divided into an input shaft 1 and an output shaft 2. The input shaft 1 and the output shaft 2 are connected to each other by a torsion bar 3 made of special alloy steel. Each end of the torsion bar 3 is inserted into the input shaft 1 and the output shaft 2, and they are secured by pins 4 and 5, respectively. A first mounting frame 6 is integrally formed with the input shaft 1. A prism-like actuator portion 7 extends downwards from the leading end of the first mounting frame 6. An adjust screw 8, which is brought into contact with a first side surface of the actuator portion 7, is fastened into the leading end of the first mounting frame 6 by means of a screw thread. A second mounting frame 9 is integrally formed with the output shaft 2 and has, at the leading end thereof, an outer case 10c for the sensing device 10 secured thereto. The sensing device 10 comprises a slide-operation-type variable resistor for transmitting a resistance value, which corresponds to the position of a prism-like lever 10a extending away from the shafts 1 and 2, when the lever 10a reciprocates within an elongated hole 10b. The lever 10a is, by a restoring spring (not shown) included in the outer case 10c, urged into a direction in which the lever 10a is brought into contact with a second side surface of the actuator portion 7.
When a steering wheel (not shown) attached to the leading end of the input shaft 1 is rotated clockwise or counterclockwise, the torsion bar 3 is twisted so that a torsional angle, which is the difference in the phase caused from the rotation, is made. The thus-made torsional angle is, as rotational torque, detected by the sensing device 10. If the input shaft 1 is rotated, for example, clockwise, the first mounting frame 6 and the actuator portion 7 are also rotated in the same direction. Thus, the actuator portion 7 is rotated in a direction in which the actuator portion 7 presses against the lever 10a of the sensing device 10, thereby moving the lever 10a within the elongated hole 10b. As a result, the sensing device 10 transmits a change in the voltage corresponding to the torsional angle. On the contrary, if the input shaft 1 is rotated counterclockwise, the first mounting frame 6 and the actuator portion 7 are also rotated in the same direction. Thus, the actuator portion 7 is rotated in a direction in which the actuator portion 7 moves apart from the lever 10a of the sensing device 10. However, since the lever 10a follows the movement of the actuator portion 7 due to the elasticity of the restoring spring, the sensing device 10 similarly transmits the voltage change which corresponds to the torsional angle. In accordance with the thus-detected rotational torque, a rotational assist power is supplied to a power assist mechanism (not shown) attached to the leading end of the output shaft 2.
With the foregoing conventional torque sensor, a dimensional error or a mounting error of the components including the sensing device 10 results in a necessity of adjusting the position, at which the actuator portion 7 and the lever 10a are in contact with each other, to be a neutral position, that is, a zero point adjustment is required. The zero point adjustment is performed in such a manner that the adjust screw 8 is rotated to deform the actuator portion 7. However, with the foregoing conventional torque sensor, the actuator portion 7 and the lever 10a are prism-like shape, thus, the contact between the actuator portion 7 and the lever 10a is performed along a surface or a line. Therefore, even if the zero point adjustment has been performed accurately, there arises a problem in that an excess change in frictional force generated between the actuator portion 7 and the lever 10a occurring during the operation inhibits the lever 10a to follow the movement of the actuator portion 7 and, therefore, hysteresis easily takes place in an output curve from the sensing device 10. If the restoring spring included in the sensing device 10 is separated from the lever 10a or the restoring spring is ruptured, the lever 10a cannot follow the movement of the actuator portion 7. Thus, detection of the rotational torque encounters a problem.